[debacle]

Because of SPAM, phishing, and malicious emails.

[fierarul]

Web browsers seem to handle malicious web pages just fine. Why is email so special?

[tomwalsham]

Primarily because the Web is a Pull medium, whereas email is Push. To get my eyeballs on the web you first have to trick me into going to your location. To get my eyeballs on Email you just have to know my easy-to-guess personal address.